PREMIXED GAS BURNING METHOD AND COMBUSTOR

Abstract: This invention aims at eliminating from a fuel-air premixing nozzle-carrying combustor the inconveniences, such as variation of flame length which is ascribed to variation of combustion rate due to variation of fuel concentration in a premixed gas; and the occurrence of combustion vibration. The present invention provides a combustion method in which a non-fuel-containing fluid (8) is supplied to a space between a premixed gas layer (2) formed by a premixed gas ejected from a premixing nozzle (1) and a high-temperature gas layer (5) formed by a part of a high temperature gas occurring due to the combustion of the premixed gas, which part of the high-temperature gas is circulated or reside in a downstream zone at an outlet portion of the premixing nozzle; and a combustor.

Description: This invention relates to a method for the combustion of a premixed gas in a combustor having a fuel-air premixing nozzle and useful for effecting low-NO x combustion and a combustor therefor. This invention can be applied to such combustors as gas turbine combustors, boilers, and furnaces for use in chemical industry. Recently, in order to reduce the NO x content of exhaust gas from gas turbine combustors and the like, there is commonly employed a combustion method in which fuel is premixed with combustion air and the resulting premixed gas is burned under lean combustion conditions. This method for the combustion of a premixed gas can eliminate areas having uneven fuel concentrations and also local areas burning at higher temperatures, resulting in a marked reduction in the amount of NO x produced. FIG. 3 illustrates a part of a burner based on the conventional method for the combustion of a premixed gas. In this figure, reference numeral 1 designates a premixing nozzle, 2 designates a premixed gas, 3 designates a flame holder, 4 designates a flame, 5 designates a circulating or staying hot gas, 6 designates a pilot nozzle, and 7 designates a pilot fuel. The premixed gas 2 injected through the premixing nozzle 1 into the combustion chamber burns so as to go round the flame holder 3, thus forming downstream a circulating hot gas 5 serving for flame holding. In this method for the combustion of a premixed gas, the combustion rate of the premixed gas (i.e., the velocity at which the flame advances into the unburned mixture) becomes higher as the temperature of the premixed gas is elevated and as the fuel concentration of the premixed gas is increased. In particular, the fuel concentration may cause marked variation in the resulting combustion rate. Such variation in combustion rate causes variation in flame length and may eventually lead to the development of combustive vibration. From US-Patent 4,373,325, a combustor, primarily for a gas turbine which employs swirl flame stabilization is known. Vortex induced centrifugal forces and hot gas recirculation are employed for flame stabilization. The dual stage combustor operates in two modes, stable fuel air strata mode and low emission mode. The combustor comprises a cylindrical, elongated casing which has an upstream section and a downstream section connected by a radially extending transition section, and a liquid fuel injector concentrically disposed in the upstream casing section. The injector and the upstream casing section form an annular combustion air passage with an inlet. In the combustion air passage adjacent the inlet and extending between the upstream casing section and the injector is a set of radially oriented, equiangularly spaced, longitudinally extending swirl vanes. Fuel is introduced into the injector through a fuel line communicating through a radially extending passage in the injector with an annular passage. The annular fuel passage extends axially downstream and then angles outwardly and opens onto the exterior surface of the injector. Downstream from the fuel passage outlet is a fuel film zone in which the swirling combustion air spreads and thins the fuel into a stratified film. Where the film of fuel leaves the injector surface, it attaches to the inner boundary of the combustion air annulus. The injector has a second, annular, radially and axially extending slot through which air is discharged to drive the combustor into the low emission operating mode. The air discharged through the second slot forms a swirling or rotating annulus which isolates the hot gases in a recirculation zone from the fuel. Initial air and fuel are discharged separately and form a stratified fuel-air charge. Initial ignition is effected by the hot gases in the inner recirculation zone. Thus, fuel and air are mixed after the fuel is discharged into the nozzle portion which also contains the second outlet for air. The fuel-gas mixture is formed with the injected air (neutral gas). The present invention has been made for the purpose of solving the above-described problems encountered with the known method and combustor and has for its object the provision of a method and combustor for the combustion of a premixed gas which can minimize the development of combustive vibration due to variation in the fuel concentration of the premixed gas. By the invention as defined in the appended claims a neutral fluid is fed between the premixed gas layer and the hot gas, so that a flame is not propagated directly from the hot gas to the premixed gas. Instead, a mid-temperature gas is generated by the mixing and diffusion of the hot gas and the neutral fluid. Thereafter, the premixed gas is burned as a result of the mixing and diffusion of the mid-temperature gas and the premixed gas. The mechanism by which the premixed gas is burned is such that the hot gas and the premixed gas intermingle with each other, during this process the premixed gas is heated by the heat transferred from the hot gas, and combustion is initiated when the combustion starting temperature is reached. Thus, according to the present invention, the premixed gas is burned under mixing and diffusion rate control, as contrasted with an ordinary premixed flame in which combustion is effected by flame propagation. Consequently, it is possible to achieve combustion which is little affected by the fuel concentration of the premixed gas. As used herein, "diffusion" means that the hot gas and the premixed gas intermingle with each other, and "rate control" means the controlling of the rate of such diffusion. "Diffusion rate control" means the controlling of the rate at which the hot gas and the premixed gas intermingle with each other. Moreover, since the hot gas and the neutral fluid are mixed to generate a mid-temperature gas which is then mixed with the premixed gas, the flame temperature in this mixing region is low and the production of NO x in this region is minimized. Furthermore, since the premixed gas is burned under diffusion rate control, the main flame is long, and heat generation or heat generation quotient can be spread in space, so that combustive vibration can be prevented. The reason why combustive vibration can be prevented is that the combustion pressure is reduced by dispersion of the heat generation and, moreover, the variation of flame length relative to the total flame length becomes smaller as a result of the prolongation of the flame. As used herein, "heat generation quotient" means the per area combustion (dQ/dx) of fuel (Q) with respect to the axial distance (x) of a combustor. Brief Description of the Drawings FIG. 1 is a schematic diagram illustrating an exemplary burner to which the present invention has been applied; FIG. 2 is a graphical representation of experimental data showing the effects of the present invention; and FIG. 3 is a schematic diagram illustrating a burner based on the conventional method for the combustion of a premixed gas. Best Mode for Carrying Out the Invention FIG. 1 illustrates an exemplary burner for carrying out the method of the present invention. In this figure, the same elements as shown in FIG. 3 are designated by the same reference numerals and no detailed explanation thereof is given. Referring to FIG. 1, a passage for feeding a neutral fluid 8 is formed between a premixing nozzle 1 and a pilot nozzle 6. The orifice for injecting the neutral fluid 8 into the combustion chamber is configured so as to cause the neutral fluid 8 to flow out along the inner wall surface of a flame holder 3. The neutral fluid 8 is a fluid containing no fuel and may comprise, for example, air, exhaust gas, steam or the like. In the arrangement of FIG. 1, a neutral fluid is fed between a layer of a premixed gas 2 injected through the premixing nozzle 1 and a circulating hot gas 5, so that the premixed gas 2 injected through the premixing nozzle 1 does not come into direct contact with the hot gas 5. The neutral fluid 8 is first mixed with the hot gas 5 to generate a mid-temperature gas. Then, in a downward region, this mid-temperature gas is mixed with the premixed gas 2 to form a flame 4. Thus, since the hot gas 5 does not come into direct contact with the premixed gas 2 fed through the premixing nozzle 1, it is unlikely that a flame is propagated directly from the hot gas 5 to the premixed gas 2 and ignites the premixed gas forcibly before the premixed gas diffuses sufficiently, as is the case with conventional burners. That is, the premixed gas 2 separated from the hot gas 5 by the neutral fluid 8 can be burned under diffusion rate control, so that a long main flame results. This means that the heat generation quotient can be spatially spread and small to prevent combustive vibration. FIG. 2 is a graph showing the states of combustive vibration when combustion was effected in a burner using air as the neutral fluid 8 according to the method of the present invention and in a conventional burner. In this graph, the ordinate shows the internal pressure fluctuation of the combustor which serves as a measure of combustive vibration, and the abscissa shows the proportion of the pilot fuel (i.e., the pilot fuel 7 for the formation of a diffusion flame) to the total fuel (i.e., the main fuel plus the pilot fuel). FIG. 2 indicates that, regardless of the proportion of the pilot fuel, the internal pressure fluctuation ratio is lower in the presence of a neutral fluid (the present invention) than in the absence of a neutral fluid (the prior art). That is, FIG. 2 indicates less combustive vibration in the presence of a neutral fluid, demonstrating the effects of the present invention. The present invention is not limited to the burner design illustrated in FIG. 1, but comprehends all combustors involving the formation of a circulating flow wherein a neutral fluid is fed between the circulating flow and the premixed gas. For example, the present invention can be applied to combustors having various flame-holding plates and combustors forming a circulating flow or a stagnation point in a region having an abruptly expanded flow path.

Claims: A method for minimizing combustive vibration in the combustion of a premixed gas (2) in a combustor having a fuel-air premixing nozzle (1) which has a flame holder (3) for holding a flame, said method comprising feeding a fluid (8) containing no fuel between a premixed gas layer formed by injecting said premixed gas through said premixing nozzle and a circulating hot gas (5) formed as a result of the combustion of said premixed gas, and directing the fluid (8) to flow out along a wall surface of the flame holder, whereby the hot gas forms a circulating flow in a region downstream of the outlet of said premixing nozzle. A combustor comprising: a pilot nozzle (6); a fuel-air premixing nozzle (1) for injecting a premixed fuel-air gas (2) which is disposed around the pilot nozzle coaxially with the pilot nozzle; a flame holder (3) disposed at the outlet of the fuel-air premixing nozzle to hold a flame; and a passage for feeding a fluid (8) containing no fuel which is disposed between the pilot nozzle and the fuel-air premixing nozzle, wherein said passage is arranged so as to direct the fluid (8) to flow out along a wall surface of the flame holder (3) so that said fluid flows between the premixed fuel-air gas (2) and a circulating hot gas (5) formed as a result of the combustion of the premixed gas, wherein the circulating hot gas (5) forms a circulating flow in a region downstream of the outlet of the premixing nozzle (1).

Inventors: MANDAI SHIGEMI, SATO NOBUO, TANAKA KATSUNORI, TANIMURA SATOSHI, MANDAI, SHIGEMI, SATO, NOBUO, TANAKA, KATSUNORI, TANIMURA, SATOSHI, MANDAI, SHIGEMI, TAKASAGO MACH. WORKS MITSUBISHI, SATO, Nobuo, Takasago Research &amp; Development Ctr., TANAKA, KATSUNORI, TAKASAGO MACH. WORKS MITSUBISHI, TANIMURA, Satoshi, Takasago Res. &amp; Dev. Ctr.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1994
Main classification: F23R3
IPC codes: F02C3, F23R3, F23C99, F23L7